There are communication schemes that use multiple antennas. Example of such communication schemes includes MIMO (multiple-input multiple-output) transmission, communications using antenna diversity, and communications using beam forming. (For example, see “MIMO increases the transmission rate several-fold”, published on the Internet on Dec. 13, 2007 (retrieved on Aug. 5, 2010), <URL: http://itpro.nikkeibp.co.jp/article/COLUMN/20071203/288625/>).
For such a multi-antenna communication scheme, as many antennas as required for the multi-antenna communication have been installed.
However, it is difficult to install a required number of antennas for multi-antenna communication if available installation space is limited.